The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Reinforced concrete members crack due to both applied structural loading and shrinkage and thermal deformations, which are practically inevitable and often anticipated in restrained conditions. These cracks have many negative effects on the mechanical performance and durability of reinforced concrete structures. For example, the presence of cracking reduces the load capacity and stiffness of concrete member. Cracks also provide pathways for the penetration of aggressive ions to cause concrete deterioration. Chlorides, oxygen, and carbonation agents can migrate through cracks and ultimately lead to corrosion of reinforcing steel which is the major cause of concrete deterioration world-wide, and the presence of cracking is the root cause of much of this deterioration. Therefore, the formation of cracks is a dominant form of damage in concrete materials. Hence, the development of self-healing concrete materials in which crack damage can reheal automatically to block transport of water and corrosives into the concrete and down to reinforcing steel and to regenerate a large portion of material load capacity and stiffness under mechanical loads is highly desirable.
The phenomenon of self-healing in concrete has been known for many years. It has been observed that some cracks in old concrete structures are lined with white crystalline material suggesting the ability of concrete to seal the cracks with chemical products by itself, perhaps with the aid of rainwater and carbon dioxide in air. Later, a number of researchers in the study of water flow through cracked concrete under a hydraulic gradient, noted a gradual reduction of permeability over time, again suggesting the ability of the cracked concrete to self-seal itself and slow the rate of water flow. The main cause of self-sealing was attributed to the formation of calcium carbonate, a result of reaction between unhydrated cement and carbon dioxide dissolved in water. Thus, under limited conditions, the phenomenon of self-sealing in concrete is well established. Self-sealing is important to watertight structures and to prolonging service life of infrastructure.
In recent years, there is increasing interest in the phenomenon of mechanical property recovery in self-healed concrete materials. For example, the resonance frequency of an ultra high performance concrete damaged by freeze-thaw actions, and the stiffness of pre-cracked specimens were demonstrated to recover after water immersion. In another investigation, the recovery of flexural strength was observed in pre-cracked concrete beams subjected to compressive loading at early age. In these studies, self-healing was associated with continued hydration of cement within the cracks. As in previous permeability studies, the width of the concrete cracks, found to be critical for self-healing to take place, was artificially limited using feedback controlled equipment and/or by the application of a compressive load to close the preformed crack. These experiments confirm that self-healing in the mechanical sense can be attained in concrete materials.
Deliberate engineering of self-healing in concrete was stimulated by the pioneering research of White and co-workers who investigated self-healing of polymeric material using encapsulated chemicals. A number of experiments were conducted on methods of encapsulation, sensing and actuation to release the encapsulated chemicals into concrete cracks. For example, Li et al. demonstrated that air-curing polymers released into a crack could lead to a recovery of the composite elastic modulus. The chemical release was actuated by the very action of crack formation in the concrete, which results in breaking of the embedded brittle hollow glass fibers containing the polymer. Thus, the healing action took place where it was needed. Another approach, taken by Nishiwaki et al., utilized a repair agent encapsulated in a film pipe that melts under heating. A heating device was also embedded to provide heat to the film pipe at the cracked location when an electric current is externally supplied. Yet another approach, suggested by the experiments of Bang et al. and Rodriguez-Navarro et al., used injected micro-organisms to induce calcite precipitation in a concrete crack. These novel concepts represent creative pathways to artificially inducing the highly desirable self-healing in concrete materials.
From a practical implementation viewpoint, autogenous self-healing is most attractive. Compared with other engineering materials, concrete is unique in that it intrinsically contains micro-reservoirs of unhydrated cement particles widely dispersed and available for self-healing. In most concrete and particularly in those with a low water/cement ratio, the amount of unhydrated cement is expected to be as much as 25% or higher. Upon cracking, the unhydrated cement particles are activated by contact with natural actuators present in the environment such as water or carbon dioxide. Such self-healing commonly takes the form of calcite precipitates or additional hydration products that fill cracks. Through these mechanisms, self-healing concrete materials can uniquely turn environmental deterioration agents into beneficial self-healing reagents. While typically slower acting than encapsulation techniques, autogenous self-healing offers great potential for long lasting functionality because these unhydrated cement particles are known to be long lasting in time, and is also economical when compared with chemical encapsulation or other approaches that have been suggested. As indicated above, the phenomenon of autogenous self-healing has been demonstrated to be effective in transport and mechanical properties recovery. Unfortunately, the reliability and repeatability of autogenous self-healing is unknown. The quality of self-healing is also rarely studied, and could be a concern especially if weak calcite is depended upon for mechanical strength recovery. Perhaps the most serious challenge to autogenous healing is its known dependence on tight crack width, likely less than 150 micron, which is very difficult to achieve in a consistent manner for concrete in the field. In practice, concrete crack width is dependent on steel reinforcement. However, the reliability of crack width control using steel reinforcement has been called into question in recent years. The latest version of the ACI-318 code has all together eliminated the specification of allowable crack width. Thus, a number of serious material engineering challenges await autogenous healing before this phenomenon can be relied upon in concrete structures exposed to the natural environment.
Previous researchers have engaged in limited studies in the phenomenon of concrete self-healing, the formation of self-healing products, and the necessary conditions to experience self-healing in concrete materials. These studies have resulted in identifying three general criteria which are critical to exhibit reliable autogenous self-healing: presence of specific chemical species, exposure to various environmental conditions, and small crack width. Most criteria for engaging autogenous healing in concrete material are satisfied automatically. For example, autogenous healing can occur in a variety of environmental conditions ranging from underwater to cyclic wet-dry exposures. These conditions are readily available for many infrastructure types. Second, adequate concentrations of certain critical chemical species are essential to exhibit autogenous healing mechanisms. This too, is readily available due to the chemical composition of cementitious materials and incomplete hydration, as well as the presence of CO2 in air and NaCI in seawater and deicing salt. However, it has been reported that control of crack width (below 150 μm and preferably below 50 μm) suitable for engaging autogenous healing mechanisms represents the most challenging task in the design and implementation of self-healing concrete materials and it explains why reliable formation of autogenous healing products in most concrete structures is not typically realized.
FIG. 1 illustrates the resonant frequency of single-crack mortar specimen before and after wet-dry cycles as a function of crack width. The y-axis gives the resonant frequency of preloaded specimens before and after the prescribed wet-dry exposure, normalized to the resonant frequency of uncracked (virgin) material. Therefor, 100% represents a total recovery of the resonant frequency. As seen in FIG. 1, the resonant frequency of pre-loaded specimens after 10 cyclic wet-dry exposures can recover up to 100% of the uncracked value provided that crack widths are kept below 50 microns. With an increase of crack width, however, the degree of material damage indicated by the drop in resonant frequency increases and the extent of self-healing diminishes. When the crack width exceeds 150 microns, the specimen resonant frequency remains unchanged after undergoing the wet-dry cycle exposure signifying the difficulty of repairing microstructural damage within these cracked materials. Therefore, maintaining a crack width below 150 microns, and preferably below 50 μm, is critical to enable the process of self-healing. This condition is difficult to achieve consistently, and explains why reliable formation of self-healing products in most concrete structures has not been realized. This set of material physical and chemical properties, and exposure conditions, may serve as a reference base towards systematic design of self-healing concrete.
In the referenced work above, crack widths are controlled in feedback controlled loading machines in laboratory conditions. In field conditions, crack width consistently below 150 μm, and especially below 50 μm when the composite is damaged by tensioning to 1% or more, have not been possible prior to the present disclosure. Hence robust autogenous healing under natural conditions has not been previously realized.
Accordingly, the present disclosure provides a self-healing cementitious composite that meets these desirable features. This fiber-reinforced cementitious composite is deliberately engineered to possess self-controlled and highly reliable tight crack width that does not depend on steel reinforcement or structural dimension. Instead, the fibers used in the composite are tailored to work with a mortar matrix in order to suppress localized brittle fracture in favor of distributed microcrack damage with highly controlled crack width, even when the composite is tensioned to several percent strain. Therefore, autogenous self-healing can occur under a variety of environmental conditions when composite is damaged. The composite comprises hydraulic cement, water, sand, fly ash, water reducing agent, and short discontinuous fiber that are mixed to form a mixture having reinforcing fiber uniformly dispersed and having preferable flowability. The mixture is then cast into a mold with desired configuration and cured to form composite.
An object of the present disclosure is to provide a means of achieving self-healing in a fiber-reinforced cementitious composite by embedding self-controlled tight crack width and high tensile ductility intrinsically into the composite.
Another object of the present disclosure is to provide selection criteria for reinforcing fibers to be used in production of self-healing cementitious composite that desired magnitude of self-controlled crack width and ductility in tension can be achieved at low fiber content. A feature of the present disclosure is the use of micromechanics parameters that describe fiber and interface properties to differentiate acceptable fiber from unacceptable fiber.
Yet another object of the present disclosure is to provide fiber-reinforced cementitious products having self-healing behavior under a variety of environmental exposures even when it is tensioned to several percent.
Still another object of the present disclosure is to provide a self-healing material for structural member in construction applications.
In practicing one embodiment of the present disclosure, the binder comprises a hydraulic cement, such as Type I Portland cement. The weight ratio of water to cement is within the range of 0.50 to 0.80. The weight ratio of sand to cement is within the range of 0.8 to 1.0. The discontinuous reinforcing fiber is polyvinyl alcohol with a diameter within the range of 30-60 micrometer and is present from about 1.5% to 3.0% by volume of the composite.
The present disclosure also provides a self-healing fiber-reinforced cementitious composite exhibiting significant multiple cracking when stressed in tension with at least 1% tensile strain and meanwhile having the crack width below 150 micrometer and preferably below 50 μm.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.